1. Field of the Invention
The present invention relates to a stencil making device for perforating a stencil sheet supplied from a rolled stencil sheet.
2. Description of the Related Art
FIG. 7 illustrates a conventional stencil making device. A rolled base sheet S for stencil printing (hereinafter referred to as a stencil sheet S) is supported in a master holder 106. The stencil sheet S supplied from the rolled stencil sheet S is in a shape of a continuous strip. The stencil sheet S changes conveying direction while turning on a set guide shaft 100. Then, the sheet is sandwiched between a thermal head 101 and a platen roller 102, and subsequently held between an upper roller 104 and a lower roller 105 after turning on a roller 103, and then conveyed to an initial stop position.
When the stencil sheet S is conveyed, tension in an opposite direction to the conveying direction is exerted on the stencil sheet S by a back-tension unit 107 disposed to the master holder 106, so that a portion of the sheet sandwiched between the thermal head 101 and the platen roller 102 does not wrinkle. The rolled stencil sheet S is attached to the back tension unit 107.
While being perforated by the thermal head 101, the stencil sheet S is conveyed only by a driving force of the platen roller 102 holding the sheet on the thermal head 101. The perforated stencil sheet S is guided downward by the roller 103 to move to a reservoir box 108. The perforated stencil sheet S is stored in the reservoir box 108 until the perforation by the thermal head 101 finishes.
On completion of perforating, the thermal head 101 moves upward to release the stencil sheet S held on the platen roller 102. The perforated stencil sheet S is conveyed by the upper and the lower rollers 104, 105 until a leading end thereof reaches a clamp plate 110 of a printing drum 109. The leading end of the stencil sheet S is held on the printing drum 109 by the clamp plate 110. When the printing drum 109 rotates simultaneously with rotation of the upper and the lower rollers 104, 105, the stencil sheet S is conveyed and wound around the circumferential surface of the printing drum 109. After the sheet is wound around the drum, a cutter unit 111 cuts the stencil sheet S.
The present device combines stencil printing function with the stencil making function. A part of the constitution for stencil printing is not illustrated in the drawings. A part of a circumferential wall of the printing drum 109 is ink-permeable. Ink supplying means is disposed inside the printing drum 109 for supplying ink to an inner surface of the circumferential wall. Pressing means is disposed under the printing drum 109 for pressing printing sheet against the printing drum 109. When the printing drum 109 rotates, the printing sheet is supplied between the printing drum 109 and the pressing means at a predetermined timing, so that the printing sheet is pressed against an image area of the stencil sheet S. The ink supplied to the inner surface of the printing drum 109 passes through the circumferential wall of the printing drum 109, and then transferred to the printing sheet through perforations of the stencil sheet S to form an image thereon.
According to the conventional stencil making device as illustrated in FIG. 7, the back tension unit 107 comprises a silicon oil damper. The rolled stencil sheet S is perforated by a perforating section comprising the platen roller 102 and the thermal head 101, and conveyed to the downstream side. Then, the back tension unit 107 exerts back tension on the stencil sheet S held between the back tension unit 107 and the perforating section.
The tension changes according to a diameter of the rolled stencil sheet S. The tension is small when the diameter of the rolled stencil sheet S is long, and it increases as the diameter decreases. Torque required to rotate the silicon oil damper for exerting the back tension is constant. A product of the tension exerted on the stencil sheet unrolled from the most outer portion of the rolled stencil sheet and the diameter of the rolled stencil sheet is balanced with torque required to rotate the silicon oil damper. Accordingly, when the diameter of the rolled stencil sheet decreases, the tension exerted on the unrolled stencil sheet increases.
The increased tension exerted on the stencil sheet may cause a slip between the platen roller 102 and the stencil sheet S. If such a slip occurs, the stencil sheet moves in shorter distance than rotational distance of the platen roller 102. The slip results in shortening the size of the perforated area in the stencil sheet than a predetermined size (a size of an original), thereby causing shrinkage in print-image.
If torque of the silicon oil damper is set at smaller level to lower the tension exerted on the stencil sheet when the diameter of the rolled stencil sheet is shorter, the stencil sheet may wrinkle in perforating when the diameter of the rolled stencil sheet is longer since the tension exerted on the stencil sheet is excessively small.
The present invention is made in view of the aforementioned problems. An object of the present invention is to provide a stencil making device capable of exerting appropriate tension on the stencil sheet S in perforating to avoid wrinkling the stencil sheet.